The invention involves the integration of steam reforming of methane and light hydrocarbons with a cogeneration power plant, resulting in the production of synthesis gas, synthesis gas by-products, power and steam, with an improved thermal efficiency and lower investment and operating costs than a non-integrated process.
Steam reforming of light hydrocarbons, to produce hydrogen and carbon monoxide, as shown in (1), is a widely used commercial process. EQU CH.sub.4 +H.sub.2 O.fwdarw.CO+3 H.sub.2 .DELTA.H=206 kJ/mol(1)
Due to the presence of excess steam, some of the carbon monoxide and steam react simultaneously as indicated by the water gas shift reaction (2). EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2 ( 2)
In the refining industry, steam reforming is a major component of most hydrogen production complexes. Approximately 90% of the hydrogen from a hydrogen plant is produced directly by steam reforming. The remaining 10% is produced via the water-gas shift process which requires CO produced in the reformer. Steam reforming is also an integral component in methanol production from natural gas (3) as well as in Fischer Tropsch processes (4) EQU CO+2 H.sub.2 .fwdarw.CH.sub.3 OH (3) EQU CO+2 H.sub.2 .fwdarw.1/n (CH.sub.2).sub.n +H.sub.2 O (4)
In response to increasing environmental concerns, the demand for hydrogen and methanol are expected to increase, leading to a need for additional steam reforming capacity. Therefore, an integrated process, resulting in improved efficiency and, hence, lower utility costs, is a timely and attractive option.
Steam reforming is traditionally carried out in multitubular fixed bed reactors which are heated on the outside in a furnace, e.g. by burning fuel such as methane and propane, to supply heat for the reaction. Since steam reforming is a highly endothermic reaction, the amount of fuel, the size of the heat transfer surface required and low thermal efficiency make steam reforming in multitubular reactors very expensive. Furthermore, furnace temperatures are very high such that expensive heat-resistant alloys are required in tubular reactors.
In addition, typical multitubular steam reforming reactors operate at a high steam to carbon ratio (S/C) of 4 to 6. A high steam to carbon ratio is needed for optimal reformer conversion of methane and a large amount of excess steam is also needed to suppress coke formation. Therefore, steam production must be factored into the cost. One approach for eliminating the costly heat transfer surfaces is through the use of two communicating fluid beds, either of which could be an upflow or downflow fixed bed, a fast fluid bed or a circulating fluid bed. In such a design, the reforming catalyst is heated directly, via combustion of fuel gas, in one of the fluid beds, and then the hot catalyst is conveyed to the other fluid bed in which the steam reforming reaction is carried out. In this way, the heat gained in the bed in which the combustion is carried out can be transferred directly to the reformer section supplying the required sensible heat and endothermic reaction heat for reaction (1). The recycling of the reforming catalyst to a combustion zone also regenerates the catalyst by burning off any coke formed during the reforming reaction. Since the continuous regeneration eliminates concerns over continuous coke build-up and, hence, permanent catalyst deactivation, lower steam to carbon ratios can be used resulting in further utility savings.
One major obstacle to such a design is the fact that steam reforming is normally carried out under pressure (150-400 psig) and, therefore, the air required for combustion and, hence, catalyst heating, must be compressed to preserve the pressure balance in the catalyst circulation loop. The cost of energy required for this compression is very high and, to some degree, off-sets the improved heat transfer benefits relative to the traditional non-contact heat transfer. A portion of the energy expended to compress the external gases sent to the combustor/regenerator can potentially be recovered by expanding the hot, pressurized gases exiting the combustor/regenerator, after separation from the solids, to a turbine to produce power. The inability of conventional turbines to operate at high temperatures (&gt;1400.degree. F.) with entrained particulates, due to excessive turbine blade erosion, provides a second major process obstacle. Filtration of hot gases is an option only if the flue gas is first cooled to a temperature for which commercial catalyst filters are available. Cooling via some external medium entails a further reduction in the thermal efficiency. At these low temperatures very little net energy is gained in excess of the energy required for compression which leads to a high investment cost and a loss in thermal efficiency.